Lithium metal dispersion in secondary battery anodes

ABSTRACT

The present invention is a secondary battery having a high specific capacity and good cycleability, and that can be used safely. The secondary battery is manufactured to include an anode formed from a host material capable of absorbing and desorbing lithium in an electrochemical system such as a carbonaceous material, and lithium metal dispersed in the host material. The freshly prepared anodes of the invention are combined with a positive electrode including an active material, a separator that a separates the positive electrode and the negative electrode, and an electrolyte in communication with the positive electrode and the negative electrode. The present invention also includes a method of preparing a freshly prepared anode and a method of operating a secondary battery including the anode of the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to commonly owned copendingprovisional application Serial No. 60/257,994, filed Dec. 22, 2000, andclaims the benefit of the earlier filing date of this application under35 U.S.C. §119(e).

FIELD OF THE INVENTION

[0002] The present invention relates to secondary batteries having highspecific capacities and particularly to anodes for secondary batteriescomprising a host material such as a carbonaceous material capable ofabsorbing and desorbing lithium in an electrochemical system and lithiummetal dispersed in the host material.

BACKGROUND OF THE INVENTION

[0003] Lithium and lithium-ion secondary or rechargeable batteries haverecently found use in certain applications such as in cellular phones,camcorders, and laptop computers, and even more recently, in largerpower applications such as in electric vehicles and hybrid electricvehicles. It is preferred in these applications that the secondarybatteries have the highest specific capacity possible but still providesafe operating conditions and good cycleability so that the highspecific capacity is maintained in subsequent recharging and dischargingcycles.

[0004] Although there are various constructions for secondary batteries,each construction includes a positive electrode (or cathode), a negativeelectrode (or anode), a separator that separates the cathode and anode,and an electrolyte in electrochemical communication with the cathode andanode. For secondary lithium batteries, lithium ions are transferredfrom the anode to the cathode through the electrolyte when the secondarybattery is being discharged, i.e., used for its specific application.During this process, electrons are collected from the anode and pass tothe cathode through an external circuit. When the secondary battery isbeing charged or recharged, the lithium ions are transferred from thecathode to the anode through the electrolyte.

[0005] Historically, secondary lithium batteries were produced usingnon-lithiated compounds having high specific capacities such as TiS₂,MoS₂, MnO₂ and V₂O₅, as the cathode active materials. These cathodeactive materials were coupled with a lithium metal anode. When thesecondary battery was discharged, lithium ions were transferred from thelithium metal anode to the cathode through the electrolyte.Unfortunately, upon cycling, the lithium metal developed dendrites thatultimately caused unsafe conditions in the battery. As a result, theproduction of these types of secondary batteries was stopped in theearly 1990's in favor of lithium-ion batteries.

[0006] Lithium-ion batteries typically use lithium metal oxides such asLiCoO₂ and LiNiO₂ as cathode active materials coupled with acarbon-based anode. In these batteries, the lithium dendrite formationon the anode is avoided thereby making the battery safer. However, thelithium, the amount of which determines the battery capacity, is totallysupplied from the cathode. This limits the choice of cathode activematerials because the active materials must contain removable lithium.Furthermore, the delithiated products corresponding to LiCoO₂ and LiNiO₂that are formed during charging (e.g. Li_(x)CoO₂ and Li_(x)NiO₂ where0.4<x<1.0) and overcharging (i.e. Li_(x)CoO₂ and Li_(x)NiO₂ where x<0.4)are not stable. In particular, these delithiated products tend to reactwith the electrolyte and generate heat, which raises safety concerns.

SUMMARY OF THE INVENTION

[0007] The present invention is a secondary battery having a highspecific capacity and good cycleability and that operates safely. Inaccordance with the invention, the freshly prepared, secondary batteryincludes an anode that is formed of a host material capable of absorbingand desorbing lithium in an electrochemical system and lithium metaldispersed in the host material. Preferably, the lithium metal is afinely divided lithium powder and more preferably has a mean particlesize of less than about 20 microns. The host material comprises one ormore materials selected from the group consisting of carbonaceousmaterials, Si, Sn, tin oxides, composite tin alloys, transition metaloxides, lithium metal nitrides and lithium metal oxides. Preferably, thehost material comprises a carbonaceous material and more preferablycomprises graphite.

[0008] The freshly prepared, secondary batteries of the inventioninclude a positive electrode including an active material, a negativeelectrode comprising a host material capable of absorbing and desorbinglithium in an electrochemical system and lithium metal dispersed in thehost material, a separator separating the positive electrode and thenegative electrode and an electrolyte in communication with the positiveelectrode and the negative electrode. Preferably, the cathode activematerial is a compound that can be lithiated at an electrochemicalpotential of 2.0 to 5.0 V versus lithium. For example, the cathodeactive material can be MnO₂V₂O₅ or MoS₂, or a mixture thereof. Thelithium metal in the anode is preferably a finely divided lithium powderand more preferably has a mean particle size of less than about 20microns. The host material comprises one or more materials selected fromthe group consisting of carbonaceous materials, Si, Sn, tin oxides,composite tin alloys, transition metal oxides, lithium metal nitridesand lithium metal oxides. Preferably, the host material in the negativeelectrode comprises a carbonaceous material and, more preferably,comprises graphite. The amount of lithium metal present in the negativeelectrode is preferably no more than the maximum amount sufficient tointercalate in, alloy with, or be absorbed by the host material in thenegative electrode. For example, if the host material is carbon, theamount of lithium is preferably no more than the amount needed to makeLiC₆.

[0009] The present invention also includes a method of preparing afreshly prepared anode for a secondary battery that includes the stepsof providing a host material that is capable of absorbing and desorbinglithium in an electrochemical system, dispersing lithium metal in thehost material and forming the host material and the lithium metaldispersed therein into an anode. The lithium metal and the host materialis preferably mixed together with a non-aqueous liquid to produce aslurry and then applied to a current collector and dried to form theanode. Alternatively, the anode can be formed by chemical means byimmersing the host material in a suspension of lithium metal in anon-aqueous liquid, and then formed into an anode.

[0010] The present invention further includes a method of operating asecondary battery. First, a freshly prepared, secondary battery isprovided that includes a positive electrode including an activematerial, a negative electrode comprising a host material capable ofabsorbing and desorbing lithium in an electrochemical system and lithiummetal dispersed in the host material, a separator for separating thepositive electrode and the negative electrode, and an electrolyte incommunication with the positive electrode and the negative electrode. Inparticular, the secondary battery is manufactured with lithium metaldispersed in the host material of the anode. The freshly assembledbattery is in a charged state and more preferably is in a fully chargedstate (with all the removable lithium present in the anode of thefreshly prepared battery). The freshly prepared secondary battery isinitially discharged by transmitting lithium ions from the negativeelectrode to the positive electrode through the electrolyte. Thesecondary battery can then be charged or recharged by transmittinglithium ions from the positive electrode to the negative electrodethrough the electrolyte and then discharged again by transmittinglithium ions from the negative electrode to the positive electrodethrough the electrolyte. The charging and discharging steps can occurfor numerous cycles while maintaining the high specific capacities ofthe cathode active materials and maintaining safe operating conditions.

[0011] These and other features and advantages of the present inventionwill become more readily apparent to those skilled in the art uponconsideration of the following detailed description and accompanyingdrawing, which describe both the preferred and alternative embodimentsof the present invention.

BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 illustrates a simplified secondary battery constructionincluding a cathode, anode, separator and electrolyte, in accordancewith the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0013] In the drawings and the following detailed description, preferredembodiments are described in detail to enable practice of the invention.Although the invention is described with reference to these specificpreferred embodiments, it will be understood that the invention is notlimited to these preferred embodiments. But to the contrary, theinvention includes numerous alternatives, modifications and equivalentsas will become apparent from consideration of the following detaileddescription and accompanying drawing.

[0014] As illustrated in FIG. 1, the present invention is a secondarybattery 10 that comprises a positive electrode or cathode 12, a negativeelectrode or anode 14, a separator 16 for separating the positiveelectrode and the negative electrode, and an electrolyte inelectrochemical communication with the positive electrode and thenegative electrode. The secondary battery 10 also includes a currentcollector 20 that is in electrical contact with the cathode and acurrent collector 22 that is in electrical contact with the anode. Thecurrent collectors 20 and 22 are in electrical contact with one anotherthrough an external circuit (not shown). The secondary battery 10 canhave any construction known in the art such as a “jelly roll” or stackedconstruction.

[0015] The cathode 12 is formed of an active material, which istypically combined with a carbonaceous material and a binder polymer.The active material used in the cathode 12 is preferably a material thatcan be lithiated at a useful voltage (e.g. 2.0 to 5.0 V versus lithium).Preferably, non-lithiated materials such as MnO₂, V_(2 O) ₅ or MoS₂, ormixtures thereof, can be used as the active material, and morepreferably, MnO₂ is used. However, lithiated materials such as LiMn₂O₄that can be further lithiated can also be used. The non-lithiated activematerials are preferred because they generally have higher specificcapacities than the lithiated active materials in this construction andthus can provide increased power over secondary batteries that includelithiated active materials. Furthermore, because the anode 14 includeslithium as discussed below, it is not necessary that the cathode 12include a lithiated material for the secondary battery 10 to operate.The amount of active material provided in the cathode 12 is preferablysufficient to accept the removable lithium metal present in the anode14. For example, if MnO₂ is the cathode active material, then one moleof MnO₂ is preferably present in the cathode 12 per mole of lithium inthe anode 14 to produce LiMnO₂ in the cathode upon discharge.

[0016] When cathode active materials are used that can be lithiated suchas those described above, the removable lithium that is cycled in thebattery is fully provided by the anode 14 and the battery is assembledor prepared in a fully charged state, as is preferred. Nevertheless, thecathode 12 can also include a minor amount of one or more lithiatedactive materials (e.g. LiCoO₂ or LiNiO₂) that do not further absorblithium at a voltage between 2.0V and 5.0V and the battery can still beprovided in a primarily charged state. In this event, the cathodepreferably has less than 50% (molar) and more preferably less than 10%(molar) of the lithiated material (e.g. LiCoO₂ or LiNiO₂) as the activematerial. Because LiCoO₂ and LiNiO₂ do not further absorb lithium, thepresence of these materials in the cathode 12 does not reduce the amountof cathode active material needed to accept the removable lithium fromthe anode 14.

[0017] The anode 14 is formed of a host material 24 capable of absorbingand desorbing lithium in an electrochemical system with lithium metal 26dispersed in the host material. For example, the lithium present in theanode 14 can intercalate in, alloy with or be absorbed by the hostmaterial when the battery (and particularly the anode) is recharged. Thehost material includes materials capable of absorbing and desorbinglithium in an electrochemical system such as carbonaceous materials;materials containing Si, Sn, tin oxides or composite tin alloys;transition metal oxides such as CoO; lithium metal nitrides such asLi_(3−x)Co_(x)N where 0<x<0.5, and lithium metal oxides such asLi₄Ti₅O₁₂. The lithium metal 26 is preferably provided in the anode 14as a finely divided lithium powder. In addition, the lithium metal 26preferably has a mean particle size of less than about 20 microns, morepreferably less than about 10 microns. The lithium metal can be providedas a pyrophoric powder or as a stabilized low pyrophorosity powder,e.g., by treating the lithium metal powder with CO₂.

[0018] The anode 14 is typically capable of reversibly lithiating anddelithiating at an electrochemical potential relative to lithium metalof from greater than 0.0 V to less than or equal to 1.5. If theelectrochemical potential is 0.0 or less versus lithium, then thelithium metal will not reenter the anode 14 during charging.Alternatively, if the electrochemical potential is greater than 1.5 Vversus lithium then the battery voltage will be undesirably low.Preferably, the amount of lithium metal 26 present in the anode 14 is nomore than the maximum amount sufficient to intercalate in, alloy with,or be absorbed by the host material in the anode 14 when the battery isrecharged. For example, if the host material 24 is carbon, the amount oflithium 26 is preferably no more than the amount sufficient to makeLiC₆. In other words, the molar ratio of lithium to carbon in the anodeis preferably no more than 1:6.

[0019] In accordance with the invention, the anode 14 can be prepared byproviding a host material that is capable of absorbing and desorbinglithium in an electrochemical system, dispersing lithium metal in thehost material, and forming the host material and the lithium metaldispersed therein into an anode. Preferably, the lithium metal and thehost material are mixed with a non-aqueous liquid such astetrahydrofuran (THF) and a binder, and formed into a slurry. The slurryis then used to form the anode 14, for example, by coating the currentcollector 22 with the slurry and then drying the slurry. The lithiummetal can also be provided in the anode by immersing the host materialin a suspension containing lithium metal in a non-aqueous liquid such ahydrocarbon solvent (e.g. hexane). The lithium metal used in thesuspension is preferably a finely divided lithium powder as discussedabove. The host material can be formed into the shape of the anode andthen dipped into the lithium metal suspension or it can be combined withthe lithium metal suspension to form a slurry and then applied to thecurrent collector and dried to form the anode. The non-aqueous liquidused to form the suspension can be removed by drying the anode (e.g. atan elevated temperature). No matter what method is used, the lithiummetal is preferably distributed as well as possible into the hostmaterial. Accordingly, as discussed above, the lithium metal 26preferably has a mean particle size of less than about 20 microns, morepreferably less than about 10 microns.

[0020] The host material 24 in the anode 14 can include one or morematerials capable of absorbing and desorbing lithium in anelectrochemical system such as carbonaceous materials; materialscontaining Si, Sn, tin oxides or composite tin alloys; transition metaloxides such as CoO; lithium metal nitrides such as Li_(3−x)Co_(x)N where0<x<0.5; and lithium metal oxides such as Li₄Ti₅O₁₂. Preferably, asmentioned above, the host material 24 preferably includes graphite. Inaddition, the host material 24 preferably includes a small amount ofcarbon black (e.g. less than 5% by weight) as a conducting agent.

[0021] As shown in FIG. 1, the cathode 12 is separated from the anode 14by an electronic insulating separator 16. Typically, the separator 16 isformed of a material such as polyethylene, polypropylene, orpolyvinylidene fluoride (PVDF).

[0022] The secondary battery 10 further includes an electrolyte that isin electrochemical communication with the cathode 12 and anode 14. Theelectrolyte can be non-aqueous liquid, gel or solid and preferablycomprises a lithium salt, e.g., LiPF₆. The electrolyte is providedthroughout the battery 10 and particularly within the cathode 12, anode14 and separator 16. Typically, the electrolyte is a liquid, and thecathode 12, anode 14 and separator 16 are porous materials that aresoaked in the electrolyte to provide electrochemical communicationbetween these components.

[0023] As mentioned above, the battery 10 includes current collectors 20and 22, which are used to transmit electrons to an external circuit.Preferably, the current collector 20 is made of aluminum foil andcurrent collector 22 is made of copper foil.

[0024] The battery 10 of the invention can be prepared by methods knownin the art and preferably has a layer thickness within the followingranges (from left to right in FIG. 1): Layer thickness Current collector(20) 20-40 μm Cathode (12) 70-100 μm Separator (16) 25-35 μm Anode (14)70-100 μm Current collector (22) 20-40 μm

[0025] The battery 10 also includes an electrolyte dispersed throughoutthe cathode 12, anode 14 and separator 16, and a casing (not shown).

[0026] In operation, the freshly prepared secondary battery 10 isinitially in a charged state, more preferably a fully charged state, andis initially discharged by transmitting lithium ions from the anode 14to the cathode 12 through the electrolyte. At the same time, electronsare transmitted from the anode 14 to the cathode 12 through the currentcollector 22, the external circuit, and the current collector 20. Thesecondary battery 10 can then be charged or recharged by transmittinglithium ions from the cathode 12 to the anode 14 through the electrolyteand then discharged again as discussed above. The charging anddischarging steps can occur for numerous cycles while maintaining thehigh specific capacities of the cathode active materials and maintainingsafe operating conditions.

[0027] The secondary battery 10 can be used for various types ofapplications. For example, the secondary battery can be used in portableelectronics such as cellular phones, camcorders, and laptop computers,and in large power applications such as for electric vehicles and hybridelectric vehicles.

[0028] The present invention provides secondary batteries having a highspecific capacity, safe operating conditions and good cycleability. Inparticular, because lithium metal is provided in the anode,non-lithiated materials can be used as the preferred cathode activematerial in the secondary battery. These non-lithiated materials havehigher specific capacities than the lithiated materials presently usedin lithium-ion batteries. Unlike traditional lithium secondary batterieshaving non-lithiated cathode active materials and metallic lithiumanodes, it has been discovered that secondary batteries produced usingnon-lithiated cathode active materials combined with the anodes of theinvention operate safely and do not generate lithium dendrites uponcycling. Furthermore, the secondary batteries of the present inventionare safer to operate than lithium-ion batteries, which become unstablewhen lithium is removed from the cathode during charging. In particular,because the cathode active material in the secondary batteries of theinvention is typically in a fully charged state when the battery isfreshly prepared, it is more stable then the cathode materials used inlithium-ion batteries. Moreover, the batteries of the invention can becharged and discharged numerous times while maintaining safe operatingconditions and the high specific capacities of the cathode activematerials.

[0029] It is understood that upon reading the above description of thepresent invention and reviewing the accompanying drawings, one skilledin the art could make changes and variations therefrom. These changesand variations are included in the spirit and scope of the followingappended claims.

That which is claimed:
 1. A freshly prepared anode for a secondarybattery comprising a host material that is capable of absorbing anddesorbing lithium in an electrochemical system and lithium metaldispersed in the host material.
 2. The anode according to claim 1,wherein the lithium metal is a finely divided lithium powder.
 3. Theanode according to claim 2, wherein the lithium powder has a meanparticle size of less than about 20 microns.
 4. The anode according toclaim 1, being capable of reversibly lithiating and delithiating at anelectrochemical potential relative to lithium metal of from greater than0.0 V to less than or equal to 1.5 V.
 5. The anode according to claim 1,wherein the host material comprises one or more materials capable ofreversibly lithiating and delithiating at an electrochemical potentialversus lithium of from greater than 0.0 V to less than or equal to 1.5V.
 6. The anode according to claim 1, wherein the host materialcomprises one or more materials selected from the group consisting ofcarbonaceous materials, Si, Sn, tin oxides, composite tin alloys,transition metal oxides, lithium metal nitrides and lithium metaloxides.
 7. The anode according to claim 1, wherein the host materialcomprises a carbonaceous material.
 8. The anode according to claim 7,wherein the carbonaceous material is graphite.
 9. The anode according toclaim 8, wherein the host material further comprises carbon black. 10.The anode according to claim 1, wherein the amount of lithium metal insaid anode is no more than the maximum amount sufficient to intercalatein, alloy with, or be absorbed by the host material in said anode whenthe anode is recharged.
 11. A secondary battery including the anode ofclaim
 1. 12. A freshly prepared, secondary battery comprising: apositive electrode including an active material; a negative electrodecomprising a host material that is capable of absorbing and desorbinglithium in an electrochemical system and lithium metal dispersed in thehost material; a separator for separating the positive electrode and thenegative electrode; and an electrolyte in communication with thepositive electrode and the negative electrode.
 13. The secondary batteryaccording to claim 12, wherein the positive electrode includes acompound that can be lithiated at an electrochemical potential of 2.0 to5.0V versus lithium as the active material.
 14. The secondary batteryaccording to claim 13, wherein the active material in the positiveelectrode is selected from the group consisting of MnO₂, V₂O₅ and MoS₂,and mixtures thereof.
 15. The secondary battery according to claim 13,wherein the active material in the positive electrode comprises MnO₂.16. The secondary battery according to claim 13, wherein the activematerial in the positive electrode is LiMn₂O₄.
 17. The secondary batteryaccording to claim 12, wherein the lithium metal in said anode is afinely divided lithium powder.
 18. The secondary battery according toclaim 17, wherein the lithium metal in said anode has a mean particlesize of less than about 20 microns.
 19. The secondary battery accordingto claim 12, wherein the anode is capable of reversibly lithiating anddelithiating at an electrochemical potential relative to lithium metalof from greater than 0.0 V to less than or equal to 1.5 V.
 20. Thesecondary battery according to claim 12, wherein the host material insaid anode comprises one or more materials capable of reversiblylithiating and delithiating at an electrochemical potential versuslithium of from greater than 0.0 V to less than or equal to 1.5 V. 21.The secondary battery according to claim 12, wherein the host materialin said anode comprises one or more materials selected from the groupconsisting of carbonaceous materials, Si, Sn, tin oxides, composite tinalloys, transition metal oxides, lithium metal nitrides and lithiummetal oxides.
 22. The secondary battery according to claim 12, whereinthe host material in said anode comprises a carbonaceous material. 23.The secondary battery according to claim 22, wherein the carbonaceousmaterial is graphite.
 24. The secondary battery according to claim 23,wherein the host material further comprises carbon black.
 25. Thesecondary battery according to claim 12, wherein the amount of lithiummetal in said negative electrode is no more than the maximum amountsufficient to intercalate in, alloy with, or be absorbed by the hostmaterial in said anode when said battery is recharged.
 26. The secondarybattery according to claim 12, wherein the amount of active material insaid cathode is sufficient to accept the removable lithium metal presentin said anode.
 27. The secondary battery according to claim 12, being ina fully charged state.
 28. A method of preparing an anode for asecondary battery comprising: providing a host material that is capableof absorbing and desorbing lithium in an electrochemical system;dispersing lithium metal in the host material; and forming the hostmaterial and the lithium metal dispersed therein into an anode.
 29. Themethod according to claim 28, wherein said dispersing step comprisesmixing the lithium metal, the host material and a non-aqueous liquidtogether to form a slurry.
 30. The method according to claim 29, whereinsaid forming step comprising applying the slurry to a current collectorand drying the slurry.
 31. The method according to claim 28, whereinsaid dispersing step comprises immersing the host material in asuspension containing lithium metal and a non-aqueous liquid.
 32. Themethod according to claim 31, wherein said dispersing step comprisesimmersing the host material in a suspension of lithium metal in ahydrocarbon.
 33. The method according to claim 28, wherein saiddispersing step comprises dispersing a finely divided lithium metalpowder in the host material.
 34. The method according to claim 33,wherein said dispersing step comprises dispersing lithium metal having amean particle size of less than about 20 microns in the host material.35. The method according to claim 28, wherein said providing stepincludes providing a host material comprising one or more materialscapable of reversibly lithiating and delithiating at an electrochemicalpotential versus lithium of from greater than 0.0 V to less than orequal to 1.5 V.
 36. The method according to claim 28, wherein saidproviding step comprises providing a host material comprising one ormore materials selected from the group consisting of carbonaceousmaterials, Si, Sn, tin oxides, composite tin alloys, transition metaloxides, lithium metal nitrides, and lithium metal oxides.
 37. The methodaccording to claim 28, wherein said providing step comprises providing ahost material comprising a carbonaceous material.
 38. The methodaccording to claim 37, wherein said providing step comprises providing ahost material wherein the carbonaceous material is graphite.
 39. Themethod according to claim 38, wherein said providing step comprisesproviding a host material wherein the host material further comprisescarbon black.
 40. A method of operating a secondary battery comprisingthe steps of: (a) providing a freshly prepared, secondary batterycomprising a positive electrode including an active material, a negativeelectrode comprising a host material capable of absorbing and desorbinglithium in an electrochemical system and lithium metal dispersed in thehost material, a separator for separating the positive electrode and thenegative electrode, and an electrolyte in communication with thepositive electrode and the negative electrode; (b) initially dischargingthe secondary battery by transmitting lithium ions from the negativeelectrode to the positive electrode through the electrolyte; (c)charging the secondary battery by transmitting lithium ions from thepositive electrode to the negative electrode through the electrolyte;(d) discharging the secondary battery by transmitting lithium ions fromthe negative electrode to the positive electrode through theelectrolyte; (e) repeating steps (c) and (d).